Ranks
Ranks, or skill points as they are sometimes called, describe how well your character can perform a skill. These ranks may be the bonus provided in seeing if a check is successful or just to see what actions may be attempted. Some systems disallow you from using certain skills until you have at least one rank in it.
While often synonymous, skill points and ranks are used differently by some games. Skill points refer to a more granulated skill advancement where someone might have 3, 12 or 30 skill points in a given skill. In such cases there is no big difference between skill point levels and they advance in a linear manner, often with no or a very high "skill cap". Ranks tend to denominate a more precisely "ranked" and limited organization of competence (like military ranks), usually running from 1-5, separating broader levels of skill (for example someone might be Average (2) at a skill and someone else Very Good (4)).
The two can also be used simultaneously. For example, the game might require you to spend a certain amount of skill points to achieve a rank in a skill.